The invention concerns transmission and reception of signalling cells in the node, especially an ATM node, of a broad-band telecommunications network.
As is known, ATM (Asynchronous Transfer Mode) is a connection-oriented packet switching technique chosen by the ITU-T, the international standardisation organisation in the telecommunications field, as the xe2x80x9ctarget solutionxe2x80x9d for a broad-band multiservice network (B-ISDN). Information is transferred in the cells shown in FIG. 1, which include a 5-bit heading and a 48-bit information part. The heading fields include the VPI (Virtual Path Indicator) and the VCI (Virtual Channel Indicator), the Payload Type, the CLP (Cell Loss Priority) and the HEC heading error check with which errors of one bit can be corrected and errors of two bits can be detected.
The standard dimensions of cells allow their swift connection. Cells belonging to different connections are separated from each other with the aid of the VPI and VCI indicators. When setting up a connection, a fixed route or virtual connection is determined through the network along which the cells of the connection will be routed. In network nodes, the cells are connected according to their VPI/VCI values. The VPI/VCI values of cells are connection space specific and will thus generally change together with the connection of the VP or VC level. When the information transfer ends, the connection is disconnected.
FIG. 2 illustrates a reference model of B-ISDN connection practices, wherein the functions are divided on the one hand into layers and on the other hand into levels.
The physical layer PHY includes two partial layers, of which partial layer PM contains transfer system specific bit level tasks while the transfer system convergence layer TC attends to adapting of cells into each transfer system and to cell bounding, to heading error check of the cell and to equalisation of the cell rate.
The ATM layer offers a cell transfer service to the AAL layer. It deals only with the cell heading and its tasks are cell connection, multiplexing and de-multiplexing, cell heading generation and removal and flow control (GFC) at the UNI (User Network Interface).
The AAL (ATM Adaptation Layer) segments the frames of upper layers, places the segments in cells and again assembles the frames at the opposite end.
The user plane attends to data transfer of user applications and to the control required therein. In principle, processing of the data of this level takes place in the user""s terminal equipment only. The control plane includes telephone and connection control functions and functions related to processing of the related signalling. This level is present in those network elements only which provide connection-related services or take part otherwise in the processing of signalling.
Of the management plane divided into two parts the tasks of layer management include management of the resources and parameters of the connection practice items of its own layer as well as management of the OAM cells relating to the structural use and maintenance of the ATM layer. It is the task of plane management to co-ordinate between different levels and to manage the system as a whole.
The interface between the integrated circuits performing functions of the physical layer PHY and the ATM layer has been standardised by the ATM Forum under the names of UTOPIA, An ATM-PHY. Interface Specification, Level 1, hereinafter the abbreviation UTOPIA1 will be used, and UTOPIA, An ATM-PHY Interface Specification, Level 2, hereinafter UTOPIA2. In practice, the ATM equipment must be implemented with commercially available ATM micro circuits, which implement an interface in accordance with the UTOPIA specification. The interface has become a de facto industrial standard which is complied with by all component suppliers making integrated ATM circuits. In the UTOPIA interface, the data is defined as 8-bit or 16-bit data depending on the actual bit rate (UTOPIA level 1 and level 2). Only ATM data is transferred in the interface and it contains the control or so-called handshaking required in two-way transfer. Circuit manufacturers implement the functions of the ATM layer as masters of their circuits, because the UTOPIA specification defines the ATM layer circuits as masters and because the manufacturers can not know in what kind of environment their customers will use the circuits. Correspondingly, the circuits of the physical layer are slaves. Slaves connected to a UTOPIA bus may communicate with the master equipment only, but not with one another.
In an ATM network, OAM cells relating to the operation and maintenance of the ATM layer are transferred from one node to another and inside the node. In addition, signalling messages may be sent over lines in the same way as is also done in a narrow-band ISDN network, although due to packet switching there is less need for signalling. Signalling on ATM lines is a rather new matter and is only now beginning to be a feature of products. Signalling may end at the node, whereby the signalling cell comes and remains in the node, or of the transit type, whereby it is relayed through the node. Performance of signalling requires implementation of the so-called SAR (Segmentation and Reassembly) function belonging to the ATM adaptation layer, which makes it possible to add signalling cells to the cell flow and to separate them from the cell flow. The SAR functions used and the ATM adaptation layer are controlled by a processor.
It is a problem with processing of signalling cells how the processor will have access to the cell flow of ATM lines in practice. If one SAR card is used, how it will be able to send signalling messages to any line and to receive messages from any line.
One way of solving the problem of signalling messages exchange between the subscriber equipment and the ATM node is presented in Patent Application EP-0355797, applicant Hitachi Ltd. The principle of this solution is shown in FIG. 3. Optical subscriber lines are connected to the ATM switchswitch through subscriber line interfaces. They perform the conversion between the optical and electric signals and also add/remove cell headings. Corresponding trunk line interfaces are located on the other side of the ATM switchswitch. One bus on the trunk line side of the switchswitch, bus 41, is reserved solely for signalling channels. This line is connected to line signalling apparatus 42, which performs assembling of the payload data of ATM cells into a signalling message data frame and, correspondingly, segmentation of the data frame and its locating in the ATM cells, that is, the function of layer 3 in FIG. 2. The function corresponds to the SAR function. Processor 43 attends to processing of the message. The function is such that when a signalling cell arrives from the subscriber line, the subscriber line interface identifies the signalling cell, modifies its heading to address bus 41, that is, line signalling apparatus 42, and adds such information to the heading which indicates from which subscriber line the cell has come. In the opposite direction, the line signalling apparatus segments the data frame into cells, adds such information to cell headings which indicates for which subscriber line and for which subscriber equipment the cell is intended. The subscriber line interface receives the signalling cell and also performs an address modification for it before it is sent to the subscriber line.
The described patent application also presents an embodiment carrying out transmission of signalling messages to trunk lines and reception from them. The embodiment is shown in FIG. 4. By adding the trunk line signalling apparatus 43 on the subscriber line side of the ATM switchswitch it is possible to receive signalling cells from trunk lines and to send them to trunk lines. The solution is practicable, because signalling on a subscriber line is different from signalling on a trunk line as regards protocol stacks.
The described known method is characterised in that signalling cells arriving from a trunk line and from a subscriber line are led through an ATM switchswitch, they are released on its other side and the data is processed there, and in that outgoing signalling cells are formed on this same side and are again led through the ATM switch and further to the trunk line or subscriber line. The processing of signalling cells is thus performed in a centralised fashion in one place.
It is an advantage of this known method that centralised signalling processing can be performed with an efficient processor, whereby the signalling software may also be huge. However, it would be advantageous in some applications to perform signalling on the same side of the ATM switch as the one from which signalling cells arrive and to which they should be sent. This could of course be done in such a way that signalling is performed in a decentralised fashion, whereby there would be separate signalling processing for each line. However, this would lead to a considerably increased need for processing compared with the centralised method. The known method also requires separate signalling processes on either side of the ATM switch, if signalling should be done in both directions from the node. Another drawback of the presented method is that it is not flexibly modular. If changes are made in the number of lines, then changes must be made in the centralised signalling unit. Even though the method presents a solution to signalling on lines, not only their signalling must be performed but also an internal signalling in the ATM node as a separate process from line signalling. Internal signalling must be carried out with buses between cards, which is a very demanding method which also increases the costs. When using the known method, switching into the ATM cell flow must be made already in the line signalling apparatus, and it is very demanding and difficult to carry this out in practice.
The present invention aims at a method and switching arrangement making it possible to meet the need for signalling in either direction from the ATM node using in the minimum case just one line signalling apparatus. It should be possible with the arrangement to perform both line signalling and signalling inside the node.
The established objective is achieved with the attributes defined in the independent claims.
The invention is based on the idea to use an ATM switch to return signalling cells to their input side and to the apparatus for processing signalling cells which is located there and which performs SAR functions. According to the invention, the signalling cell processing apparatus is connected to a circuit in the ATM layer using a standard UTOPIA interface. When seen from the ATM layer circuit, the processing apparatus does not hereby differ in any way from those physical layer circuits connected to it through the UTOPIA interface which connect the optical fibres with the ATM node. Signalling cells arriving from the line are identified in the ATM layer circuit by the heading, they are picked out from the cell flow and a physical circuit tag is added to them to indicate from which input line they have arrived. Thereupon they are transmitted to the ATM switch which connects the cells to the output buffer. In the output buffer, cell headings are modified to replace the VPI/VCI information with the tag of that physical circuit, through which the signalling cell has arrived, and the old tag is replaced with a new tag, by which the ATM layer circuit will direct modified signalling cells through the UTOPIA interface to the signalling cell processing apparatus. Other cells coming from the switch have a normal VPI/VCI heading and the tag of the target physical circuit, so the ATM layer circuit is able to direct them by way of the UTOPIA interface to the correct physical circuit, which will transmit them further to the optical fibre.
In a similar manner, the signalling cell processing apparatus forms a response message by placing its data in new modified signalling cells. Instead of the VPI/VCI values it provides them with the address of the target physical apparatus and directs them through the UTOPIA interface to the ATM layer circuit. This will add before the modified signalling cell a tag indicating the cell source. The cell thereupon goes through the ATM switch to the output gate, which again modifies the cell into a signalling cell according to the standard by placing instead of the address of the physical apparatus a standard VPI/VCI value identifying the signalling cell and by replacing the old tag with a new tag by which the cell is directed over the UTOPIA interface to the correct physical apparatus, which transmits it further to the optical fibre.
The processing unit may receive signalling cells from either side of the ATM switch and irrespective of whether the signalling cells pass through the user network interface (UNI) or through the networkxe2x80x94network interface (NNI).
In this way, signalling cells are circulated through the ATM switch, so the connection into the cell flow is done already at the ATM level using already existing node circuits.